|Authors: ||M. Kanechi, A. Maekawa, Y. Nishida, E. Miyashita|
|Keywords: ||chlorophyll fluorescence, CO2 uptake rate, frequency, light fleck, quantum yield|
The effects of pulsed irradiation based-LEDs (light-emitting diodes, red and blue wavelengths, 660:455 nm = 4:1) on the growth and photosynthetic light utilization efficiency of lettuce (Lactuca sativa) leaves were studied.
Plants were hydroponically grown in an incubator at 22░C for one month after seeding under different pulse-cycled irradiations, compared with continuous light at PPFD of 200 Ámol m-2 s-1 with 16-h light/8-h dark photoperiod.
The tested pulse-cycled irradiations were of 0.5, 1, 1.3, 2.5, 5, 50, 500 Hz, and 1, 1.3, 2, 4, 10, 20 kHz frequencies, at 50% duty ratio (illuminated duration/cycle). The photosynthetic rate (Pn) was maintained relatively constant over the range of measurements at pulsed light at PPFD of 80 Ámol m-2 s-1. At PPFD of 200 Ámol m-2 s-1, Pn gradually decreased by lowering frequency below 2.5 Hz of pulsed light.
When Pn of plants grown under pulsed light was measured at the same irradiance, it was slightly higher at the measurement under pulsed light than that at the measurement under continuous light.
Chlorophyll fluorescence parameters (Fv/Fm, FvRSQUO/FmRSQUO, qP), which were measured under dark or light conditions, showed no significant difference between plants grown under pulsed light and those grown under continuous light except at the lowest frequency (0.2 Hz). The similar quantum yield (ěPSII) and electron transport rate (ETR) of Photosystem II were obtained in wide range of frequency of pulsed light, which might be an effective illumination strategy for cultivating leaf lettuce by using artificial lamps.
Flashing irradiation did not significantly change chlorophyll content.
Results suggest the effectiveness of pulsed light at 50% duty ratio on the growth of leaf vegetables that were cultivated in a closed type plant factory with the possibility of saving electricity by using intermittent illumination system with LEDs.
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